This invention relates to improvements in or relating to sound detection, in particular to a remote sound detector and a method of remote sound detection.
Acoustic signals are frequently used for detecting and locating remote objects such as guns and vehicles on a battlefield. Sensitive directional acoustic receivers arc able to determine the direction of acoustic signals emanating from powerful remote sources with considerable accuracy. This enables the determination of the position of the source from which the acoustic signal is emanating, provided that there are no intervening obstacles to attenuate or diffract the acoustic signal.
There are several known laser techniques used for measuring the velocity of air or airflow from a remote position, for example the measurement of air velocity and airflow profile in a wind tinnel using a laser beam passed into the airflow through a window. A typical technique employs a laser to produce two beams intersecting at an angle at a point within the airflow, producing interference fringes in the region where the beams overlap. Thus particles propelled by the airflow through the fringes produce scattered light which is modulated periodically by the passage of the particles through the fringes. The frequency of modulation, detected by an optical receiver, provides a measurement of the particle velocity and hence the airflow velocity. This technique is difficult to employ effectively at a range of more than a few meters from the laser source. Therefore a different technique is required to make measurements at longer ranges.
One such technique is to employ a single frequency continuous wave laser formed into a beam by a telescope which is focussed on a remote point in the atmosphere where the air velocity is to be measured. Aerosols and dust in the atmosphere scatter a small proportion of the light from the focal region back to the telescope where it is focussed into a parallel beam. The return beam is separated from the outgoing beam by a polarisation switch, typically consisting of a polarising prism and a quarter-wave plate. Light from the laser is polarised in a direction which allows it to pass through the polarising prism in the outgoing direction and it then passes through the quarter-wave plate where it is converted into right-hand circular polarisation. When the beam is scattered by particles at the focal point, the light is substantially converted into left-hand circular polarisation, so that, on its return to the telescope, it passes through the quarter-wave plate and is substantially converted to linear polarisation, but with a direction of polarisation at right angles to that of the outgoing beam. The return beam is reflected by the polarising prism onto a photo detector with a small proportion of the outgoing beam which is reflected from the far face of the quarter-wave plate which returns along the same path as the return beam and is then reflected by the prism onto the photo detector where it forms fringes by interference with the returned beam.
If the particles at the focal point of the beam move with a component of velocity along the beam direction, then the light of the return beam is Doppler shifted with respect to the light of the outgoing beam and the interference fringes move across the photo detector with a frequency equal to the Doppler shift. This causes an alternating photo current to appear on the photo detector at the Doppler frequency. The current is detectable by a suitable signal processing technique such as fast Fourier transform analyser and the velocity of the air passing through the focal point is therefore measurable by determining the frequency of the Doppler signal identified by the fast Fourier transform analyser.
Sound passing through the focal point of the outgoing beam causes periodical variations in air velocity which can be detected if the frequency resolution of the signal analyser is high enough, enabling the sound frequency and amplitude to be determined. However, there are several reasons why the continuous wave system is inefficient in performing such measurements. Firstly, the sample volume needs to be significantly smaller than the acoustic wavelength to efficiently measure the modulation in air velocity due to the acoustic signal passing through it. Secondly, the velocity modulation due to sound, typically one millimeter per second or less, is very small in comparison with atmospheric wind speeds, typically three meters per second such that a detection system is needed which is designed specifically to measure small periodical variations in wind speed rather than the absolute wind speed.
The continuous wave system described above has a range resolution determined by the depth of focus of the laser beam, which is typically one hundred meters or more when the laser beam is focussed at a range in excess of one kilometer. Acoustic wavelengths on the other hand are typically ten meters or less, so the continuous wave system is unable to provide a spatial resolution required.
Furthermore, in practice the source of the acoustic signal is frequently hidden from observers on the ground by undulations in the terrain which prevent the acoustic signals from travelling in a straight line from the source to an observers receiver. In such conditions it is not possible to locate the position of the acoustic signal source with any accuracy and the acoustic signal may be so heavily attenuated by diffraction around intervening terrain that it is undetectable by the observers receiver.
Prior art document U.S. Pat. No. 5,424,749 (Richmond) teaches a remote sound detector comprises a transmitter operably arranged to produced a train of signals and to transmit the signals into a region of atmosphere as a beam and a receiver operably arranged to receive resultant signals from the region of atmosphere. This document also teaches a method of remote sound detecting comprises transmitting a train of signals into a region of atmosphere as a beam and receiving resultant signals from the region of atmosphere.
It is an object of the present invention to obviate or mitigate the problems associated with the prior art.
According to a first aspect of the present invention the transmitter produces a train of pulse to pulse coherent signals, the receiver is arranged to receive any resultant signals from the intersection of the beam with acoustic signals in the region of the atmosphere and a detector is operably connected to the receiver and arranged to determine the presence of acoustic signals from the phase difference between successive resultant signals. The detector may determine phase differences between immediated successive pairs of resultant signals.
A laser source may be operably arranged to produce a laser beam and the laser beam may be modulated by a modulator to produce the train of signals.
The beam may be directed to a region in the atmosphere above possible sources of acoustic signals hidden from a field of view of an observer.
An interferometer may be operably arranged to provide an interference pattern between the laser beam and each resultant signal. A photoreceiver may be operably arranged to detect and produce an output signal corresponding to changes in each interference pattern. A sampler may be operably arranged to sample the output signals from the photoreceiver and a comparator may be operably arranged to compare output signals from immediate successive pairs of outputs from the photoreceiver to produce a result. An accumulator may be operably arranged to accumulated each result and a loudspeaker may be operably arranged to reproduce an audible output of the result.
The sampler may be operably arranged to sample the output signals from the photoreceiver at different ranges to the regions and a processor may be arranged to determine the curvature of an acoustic signal wavefront from a possible source, to determine a first circle from the wavefront substantially perpendicular to the beam which intersects the acoustic signal, to calculate a second circle as for the first circle with a beam directed to a different region and to locate the possible source of acoustic signal as the point that the first and second circles join.
According to a second aspect of the present invention the method of remote sound detecting including transmitting a train of pulse coherent signals into the region of the atmosphere, receiving any resultant signals from the intersection of the beam with acoustic signals in the region of the atmosphere, and determining the presence of acoustic signals from the phase difference between successive resultant signals.
The method may also include determining the phase difference immediate successive pairs of resultant signals.
The method may include producing the train of signals by producing a laser beam and modulating the laser beam.
The method may further include directing the beam to a region in the atmosphere above possible sources of acoustic signals hidden from a field of view of an observer.
The method may include providing an interference pattern between the laser beam and each resultant signal and detecting and producing an output signal corresponding to changes between each interference pattern. The method may include sampling the output signal and comparing output signals from immediate successive pairs of output signals and producing a result. The method may further include accumulating each result and providing an audible output of the result.
The method may also include sampling the output signal at different ranges to the regions, determining the curvature of an acoustic signal wavefront from a possible source, determining a first circle from the wavefront substantially perpendicular to the beam which intersects the acoustic signal, calculating a second circle as for the first circle with a beam directed to a different region and locating the possible source of acoustic signal as the point that the first and second circles join.